Novel high performance Al2O3/poly(ether ether ketone)
nanocomposites for electronics applications
R.K. Goyal
a,*
, A.N. Tiwari b, U.P. Mulik a, Y.S. Negi
c,*
a
c
Centre for Materials for Electronics Technology (C-MET), Department of Information Technology, Govt. of India, Panchwati,
Off Pashan Road, Pune 411008, India
b
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
Polymer Science and Technology Laboratory, Department of Paper Technology, Indian Institute of Technology, Roorkee, Saharanpur Campus,
Saharanpur, U.P. 247 001, India
Abstract
This paper deals with the preparation and characterization of nanocomposites of poly(ether ether ketone) (PEEK) containing nanoaluminum oxide (n-Al2O3) filler up to 30 wt% (12 vol%) loading. Nanocomposites showed improved thermal stability, crystallization,
and coefficient of thermal expansion (CTE). Thermogravimetric analysis showed enhanced thermal stability and char yield on increasing
the n-Al2O3 loading in PEEK matrix. The peak crystallization temperature is increased up to 13 C for the nanocomposites as compared
to pure PEEK. The CTE is decreased to a value very close to the CTE of copper at 12 vol% Al2O3 loading. The CTE values obtained
were compared with the theoretical equations in the literature. The X-ray diffraction showed that PEEK crystalline structure is
unchanged with addition of n-Al2O3. The distribution of n-Al2O3 in the PEEK matrix was studied by transmission electron microscopy
and scanning electron microscopy. The results show that the prepared n-Al2O3/PEEK nanocomposites may have potential applications
in electronics.
Keywords: A. Polymer-matrix composites; PEEK; B. Thermal properties; D. X-ray diffraction; D. Transmission electron microscopy
1. Introduction
High performance polymer composites such as poly(ether ether ketone) (PEEK), polyethersulphone (PES),
polyphenylenesulphide (PPS) and polyimides reinforced
with ceramic fillers result in unique combination of thermal, mechanical and electrical properties, which make
them useful for various applications. By introducing suitable reinforcing fillers in polymers, composite properties
can be tailored to meet specific design requirements such
as low density, high strength, high stiffness, high damping,
chemical resistance, thermal shock resistance, high thermal
conductivity, low coefficient of thermal expansion (CTE)
and good electrical properties such as dielectric constant.
It is well documented that PEEK exhibits excellent thermal, mechanical, electrical properties, good moisture and
chemicals resistance [1]. Recently, its properties have been
further improved by incorporating micron size particles
such as aluminum nitride (AlN) [2,3], aluminum oxide
(Al2O3) [4], CaCO3 [5], and hydroxiapatite (HA) [6] fillers.
In the last one decade, polymer based nanocomposites containing nanofillers have been intensively investigated due to
filler’s much higher surface area to volume ratio, which
results in much higher interface between the nanofillers
and the polymer matrix as compared to conventionally
used micron size fillers and polymer matrix. Hence, a very
low loading (<5 vol%) of nanofillers is required to improve
the thermal, mechanical, optical, electrical and magnetic
properties in contrast to high loading (>20 vol%) of micron
1803
size fillers. In particular the typical micron size fillers
needed for reducing the CTE of polymers are as high as
50 vol% [7]. As a result of high filler loading, the main
advantages such as ease of processing and light weight of
polymers get lost. Therefore, the use of nanofiller in polymer composites has attracted the attention of materials scientists, technologists, and industrialists for different
applications. Nevertheless, the effect of nanofiller on properties of composites depends strongly on its shape, size,
aggregates size, surface characteristics and degree of dispersion. In order to improve properties of polymer nanocomposites, a homogeneous dispersion of the nanofillers
in the polymer matrix is essential [8–14].
There are a several hundred publications on the effect of
ceramic fillers on different polymer properties, but there is
rare literature on the effect of n-Al2O3 filler on PEEK.
However, recently Kuo et al. have studied the effect of nAl2O3 and n-SiO2 (up to 5 vol%) on PEEK’s mechanical
and thermal properties [15]. Moreover, Wang et al. have
studied the wear properties of PEEK by incorporating
SiC [16], SiO2 [17], Si3N4 [18], and ZrO2 [19] nanofillers
up to 20 wt%. Nevertheless, higher loading of fillers is
required to decrease the CTE of the polymer to avoid the
thermal stresses and to increase the thermal conductivity
of polymer to dissipate the heat generated during turning
on and turning off the electronic devices.
In view of the above, in present paper a systematic investigation of the effect of electrically insulating and thermally
conducting n-Al2O3 filler on the PEEK nanocomposites
prepared by mixing PEEK and n-Al2O3 fillers (up to
30 wt%) in alcohol medium using mechanical stirring followed by hot compression molding was studied. The density, thermal stability, melting and crystallization
behavior, CTE, and crystal structure of the nanocomposites were characterized by using density, thermogravimetric
analysis (TGA), differential scanning calorimetry (DSC),
thermomechanical analyzer, ands X-ray diffraction technique, respectively. The dispersion of the n-Al2O3 fillers
in PEEK matrix was observed by scanning electron microscopy (SEM) and transmission electron microscopy.
2. Experimental
2.1. Materials
The commercial PEEK, grade 5300PF donated by
Gharda Chemicals Ltd. Panoli, Gujarat, India under the
trade name GATONETM PEEK was used as matrix. It has
a reported inherent viscosity of 0.87 dl/g measured at a concentration of 0.5 g/dl in H2SO4. The filler used in the preparation of nanocomposites was n-Al2O3 of density 4.00 g/
cm3. It was used as supplied by Aldrich Chemical Company.
Figs. 1a and b are typical SEM micrographs of PEEK powder and n-Al2O3 powder, respectively. As received ethanol
of Merck grade was used for homogenizing the n-Al2O3
and PEEK mixture. The particle size of the PEEK determined by GALAI CIS-1 laser particle size analyzer was
Fig. 1. SEM micrographs of: (a) PEEK powder, magnification = 2 · 103;
(b) n-Al2O3 powder, magnification = 10 · 103.
ranges from 4 to 49 lm. The mean size of the PEEK particle
was 25 lm. The reported average particle size and surface
area of n-Al2O3 is 39 nm and 43 m2/g, respectively.
2.2. Nanocomposites preparation
Nanocomposites of PEEK reinforced with n-Al2O3 up to
30 wt% loading were prepared using the method described
in our previous paper [2]. Dried powder of n-Al2O3 and
PEEK were well premixed through magnetic stirring at high
stirring speed using an ethanol as medium and the resultant
slurry was dried in an oven at 120 C to remove the excess
alcohol. The pure PEEK (controlled) and nanocomposite
samples were prepared by using a laboratory hot press
under a pressure of 15 MPa at a temperature of 350 C.
3. Characterization
3.1. Density
The density of the nanocomposites prepared by taking
appropriate amount of PEEK and n-Al2O3 was increased
due to higher density of n-Al2O3 (4.00 g/cm3) as compared
1804
to pure PEEK (1.30 g/cm3). Theoretical density (qth,c) of
the nanocomposites was calculated by the rule of mixture
with no voids and no loss of fillers during processing
qth;c ¼ qm V m þ qf V f
ð1Þ
where qm, qf, Vm, and Vf is the density of matrix, density of
filler, volume fraction of matrix, and volume fraction of filler, respectively.
Experimental density (qex,c) of the PEEK nanocomposites was determined by Archimedes’s method using:
qex;c ¼ ½W air =ðW air W alcohol Þ qalcohol
ð2Þ
where Wair and Walcohol is the weight of the sample in air
and in alcohol medium, respectively. The qalcohol is the density of the alcohol medium used.
3.2. Thermogravimetric analysis (TGA)
The thermal stability of the PEEK nanocomposites was
determined on a TGA using Mettler-Toledo TGA/SDTA
851e. The samples were heated from room temperature to
1000 C at the heating rate of 10 C/min in air or nitrogen
atmosphere. The maximum decomposition temperature
(Tm), was taken as the temperature corresponding to the
maximum of the peak obtained by the first order derivative
curve. The % char yield was determined at temperature of
1000 C in nitrogen atmosphere.
3.3. Differential scanning calorimetry (DSC)
The melting and non-isothermal crystallization behavior
of PEEK nanocomposites was performed on DuPont
Instruments 910 DSC. The samples placed in aluminum
pan were first heated from 30 C to 400 C at a heating rate
of 5 C/min and soaked isothermally at 400 C for 5 min to
allow complete melting of the polymer. The samples were
then cooled to 30 C at a cooling rate of 5 C/min. Each
sample was subjected to single heating and cooling cycles
under a dry nitrogen purge.
manually ground and polished with successive finer grades
of emery papers followed by cloth (mounted on wheel) polishing to remove scratches developed during emery paper
polishing. Thus, obtained samples were called as polished
samples in the present study. The same polished samples
were also etched for 5 min in a 2% w/v solution of potassium permanganate in a mixture of 4 vol. of orthophosphoric acid and 1 vol. of water and were called as etched
samples. After polishing and etching, samples were rinsed
well in water and dried for examining the polished and
etched samples, respectively. The morphology of PEEK
and Al2O3 powder was determined by suspending powder
in an ethanol followed by dispersing on metal stub. Finally
the samples were coated with a thin layer of gold using gold
sputter coater [Polaron SC 7610] to make the sample electrically conducting. Dispersability of the n-Al2O3 filler in
the PEEK matrix was also observed using TEM (Philips
CM 30) operated at an accelerating voltage of 200 kV.
The ultra-thin section slice (100 nm thick) of the nanocomposites was cut with ultramicrotome (Leica Ultracut
UCT) at room temperature. The slices were mounted on
200-mesh copper grids and dried before the TEM
observation.
3.6. Thermo mechanical analyzer (TMA)
The out-of-plane (through thickness direction) CTE of
the nanocomposites were determined using Perkin–Elmer
DMA 7e in thermo mechanical analyzer mode. The
detailed procedure of the CTE measurement was described
elsewhere [20]. The annealed sample was heated under
pressure of 50 mN from 30 to 250 C at a heating rate of
5 C/min in argon atmosphere. The sample was then
cooled to 30 C and reheated at 5 C/min to 250 C. The
results were reported for the second run and an average
value of CTE was determined over a specific temperature
range of 30–140 C, i.e. below glass transition temperature
(Tg) of PEEK.
4. Results and discussion
3.4. X-ray diffraction measurements
XRD pattern of as molded PEEK nanocomposites was
recorded on Philips X’Pert PANalytical PW 3040/60. Nifiltered Cu Ka radiation (k = 1.54 Å) generated at 40 kV
and 30 mA was used for the angle (2h) ranged from 10
to 50. The scan step size and time per step was 0.02
and 5 s, respectively.
3.5. Morphological examination
Morphological analysis of the PEEK powder, n-Al2O3
powder and nanocomposites pellets was conducted with a
SEM (Quanta 200HV, FEI). For SEM study of nanocomposites, a small piece of the sample was cut from the pellets
and mounted in a block of acrylic based polymer resin
(DPI-RR cold cure). The obtained sample surfaces were
PEEK nanocomposites reinforced with varying weight
fraction of n-Al2O3 were prepared by hot compression
molding technique. Resulting compositions were characterized and discussed in details in this section. Table 1 showed
the properties of the PEEK matrix and Al2O3 filler. These
Table 1
Properties of PEEK and Al2O3
Material
Density (g/cc)
CTE (·106/C)
Young’s modulus (GPa)
Shear modulus (GPa)
Bulk modulus (GPa)
Poisson ratio
a
b
Experimental results.
Suppliers datasheet.
PEEK [1]
a
1.30
58a
3.6
1.3
6.2
0.40
Al2O3 [7]
4.00b
6.6
385
155
247
0.24
1805
4.2. Thermogravimetric analysis (TGA)
Table 2
Composition of n-Al2O3/PEEK nanocomposites
Al2O3 in PEEK by:
Sample code
NC-0
NC-1
NC-2
NC-5
NC-7
NC-10
NC-20
NC-30
wt%
vol%
0
1.25
2.5
5.0
7.5
10
20
30
0
0.41
0.82
1.67
2.54
3.46
7.46
12.14
properties were used to estimate the theoretical density and
CTE of the composites. Table 2 showed the weight % and
volume % of the n-Al2O3 filler added into the PEEK
matrix. From the given weight fraction of filler, volume
fraction of the filler can be determined by using:
V f ¼ W f =½W f þ ð1 W f Þ qf =qm ð3Þ
where Wf is the weight fraction of the filler.
Figs. 3 and 4 show the percentage of original weight
remaining as a function of temperature in nitrogen and
air atmosphere, respectively. The temperature of 10 wt%
loss was taken as the degradation temperature (T10) and
tabulated in Table 3. It can be seen from Table 3 that pure
PEEK has T10 in nitrogen atmosphere ðT 10;N2 Þ at 570 C
and in air atmosphere (T10,air) at 556 C, which is attributed to the decomposition of the PEEK matrix. Pure nAl2O3 powder does not show (not shown in figure) any
abrupt change in weight and only a slight (3–4%)
decrease at 500 C appears due to the loss of physisorbed
water [21].
It is observed that as the n-Al2O3 loading increases in
PEEK the degradation temperature (thermal stability) of
nanocomposites is improved significantly. The increase in
thermal stability by 14 C and 28 C was observed for the
NC-10 nanocomposites in nitrogen and air atmosphere,
respectively. However, on further increasing the n-Al2O3
loading to 30 wt% decreased the T10 value to below the
4.1. Density
Fig. 2 shows the density of the n-Al2O3 filled PEEK as a
function of n-Al2O3 content. It can be seen that the nanocomposites density increased with n-Al2O3 loading in a linear fashion due to the higher density of n-Al2O3 (4.00 g/
cm3) than that of pure PEEK (1.30 g/cm3). The experimental density of the nanocomposites is in good agreement
with the theoretical density except at 12 vol% nanoAl2O3. This might be an indication of the porosity free
samples due to good processing conditions. The experimental density of the NC-30 nanocomposite is about 1.3% lesser than theoretical density. This may be due to the
presence of voids, which is resulted from the n-Al2O3
agglomerates. During hot pressing the infiltration of melt
PEEK resin, due to very high viscosity, is difficult through
the agglomerates, hence results in voids in the final
samples.
Fig. 3. TG curves of the nanocomposites at the heating rate of 10 C/min
under nitrogen atmosphere: (a) NC-0, (b) NC-1, (c) NC-2, (d) NC-5, (e)
NC-7, (f) NC-10, (g) NC-20, and (h) NC-30.
1.7
Theoretical density
Density (g/cc)
1.6
Experimental density
1.5
1.4
1.3
1.2
0
2
4
6
8
10
Volume % of n-Al 2O3 in PEEK
12
14
Fig. 2. Density of the nanocomposites as a function of the n-Al2O3
content.
Fig. 4. TGA curves of the nanocomposites at the heating rate of 10 C/
min under air atmosphere: (a) NC-0, (b) NC-1, (c) NC-2, (d) NC-5, (e)
NC-7, (f) NC-10, (g) NC-20, and (h) NC-30.
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Table 3
Degradation temperature and char yield of the n-Al2O3/PEEK nanocomposites
Sample code
NC-0
NC-1
NC-2
NC-5
NC-7
NC-10
NC-20
NC-30
a
b
Td in air atmosphere (C)
Td in N2, atmosphere (C)
T10,aira
Tm,1
Tm,2
Tf
T 10;N2 b
Tm,1
556
580
564
567
578
584
567
580
590
590
582
588
588
588
588
590
644
670
642
658
678
695
695
688
664
744
670
692
735
736
715
710
570
578
572
570
573
584
575
580
584
592
590
592
590
594
590
590
Char yield, %
48
49
49
50
53
53
56
62
T10,air is the degradation temperature at 10 wt% loss in air atmosphere.
T 10;N2 is the degradation temperature at 10 wt% loss in nitrogen atmosphere.
580 C. As the n-Al2O3 content increased the agglomeration tendency of n-Al2O3 fillers increased, and thermal stability decreased but it is still higher than pure PEEK.
Therefore, the incorporation of n-Al2O3 in PEEK matrix
improved thermal stability of the nanocomposites in both
atmospheres. The increase in thermal stability could be
attributed to the interaction between the n-Al2O3 and
PEEK matrix, which hindered the segmental movement
of the PEEK [22].
Figs. 5 and 6 show the derivative thermogravimetric
analysis (DTG) curves of nanocomposites in nitrogen
and air atmosphere, respectively. It can be seen from
Fig. 5 that there is 6–10 C increase in maximum decomposition temperature (Tm1) in nitrogen atmosphere. The
increase in Tm1 did not vary much with increase in volume
fraction of n-Al2O3. Fig. 6 shows two decomposition stages
of PEEK nanocomposites under air atmosphere in contrast
to single decomposition stage under nitrogen atmosphere.
The lower maximum decomposition temperature (Tm1) is
probably occurred from the degradation of the PEEK molecules due to thermal energy, while the higher maximum
decomposition temperature (Tm2) is expected to be the oxidation of the degraded PEEK backbone. As shown in
Table 3, there is no significant change in Tm1. However,
Tm2 is significantly increased from 644 C for pure PEEK
(NC-0) to 695 C for NC-10. This implies that thermo-oxidative stability of nanocomposites is improved by about
50 C. Moreover, the final decomposition temperature
(Tf) in air atmosphere is increased by about 42 C from
694 C for NC-0 to 736 C for NC-10. The n-Al2O3 filler,
uniformly dispersed within the PEEK matrix, probably
interfere with degradation mechanism hence improved
the decomposition temperature.
Table 3 shows that the char yield of pure PEEK is about
48%, in agreement with a reported value [23]. This char
yield was increased to 62% for NC-30 due to the increase
in wt% of n-Al2O3, which is thermally very stable at higher
temperature. Similar trend of char yield was obtained for
micron size Al2O3 incorporated PEEK composites [4].
Fig. 5. DTG curves of the nanocomposites at the heating rate of 10 C/
min under nitrogen atmosphere: (a) NC-0, (b) NC-1, (c) NC-2, (d) NC-5,
(e) NC-7, (f) NC-10, (g) NC-20, and (h) NC-30.
Fig. 6. DTG curves of the nanocomposites at the heating rate of 10 C/
min under air atmosphere: (a) NC-0, (b) NC-1, (c) NC-2, (d) NC-5, (e)
NC-7, (f) NC-10, (g) NC-20, and (h) NC-30.
4.3. Differential scanning calorimetry (DSC)
DSC measurements were carried out to determine the
thermal properties such as melting temperature (Tm), heat
of crystallization (Hc), degree of crystallinity, onset crystallization temperature (Ton), and peak crystallization temperature (Tc) of PEEK nanocomposites. The DSC heating and
cooling curves are shown in Figs. 7 and 8, respectively.
1807
Fig. 7. DSC heating curves of the nanocomposites: (a) NC-0, (b) NC-1,
(c) NC-2, (d) NC-5, (e) NC-7, (f) NC-10, (g) NC-20, and (h) NC-30.
From the recorded heating and cooling curves, thermal
properties were calculated and tabulated in Table 4. The
crystallinity percentage of PEEK (vc) was calculated with
a value of the heat of crystallization for the 100% crystalline PEEK as 130 cal/g [2]. The crystallinity of PEEK constituent in composite was determined by:
vc ð% CrystallinityÞ ¼ DH c 100=ðDH 0c wÞ
DH 0c
ð4Þ
where
is the heat of crystallization (130 J/g) for 100 %
crystalline PEEK, and w is the mass fraction of PEEK in
the composites.
It is seen from the curves (a–h) of Fig. 7 and Table 4 that
Tm is increased by 1–6 C as the n-Al2O3 content increased
in PEEK. However, above 10 wt% the significant increase
in Tm was not observed. The similar increasing trend in
Tm was reported recently for AlN (5 lm)/PEEK [2] and
Al2O3 (8 lm)/PEEK [4] composites. However, a recent
study has shown that the addition of nano Al2O3 and nano
SiO2 decreases slightly the Tm of PEEK [15]. Lorenzo
MLD et al. reported that Tm of the PET is decreased with
the addition of untreated CaCO3 but increased with the
addition of treated CaCO3 due to good adhesion between
the filler and matrix [24]. Pingping et al. have not found significant change in the Tm of CaCO3/PET composites [25].
However, the decrease in Tm about 5–6 C of CaCO3/
PEEK composites was observed, irrespective of filler’s surface treatment [5]. It is well known that the melting point of
the polymer crystals is a function of lamellar thickness and
Fig. 8. DSC cooling curves of the nanocomposites: (a) NC-0, (b) NC-1,
(c) NC-2, (d) NC-5, (e) NC-7, (f) NC-10, (g) NC-20, and (h) NC-30.
degree of crystal perfection [26]. Therefore, the increase in
Tm, in present study, may be due to the increased crystal
size, and crystal perfection. Priya et al. reported that
change in crystal structure/morphology of composite due
to the addition of filler affect the Tm of the polymer [27].
This factor may be ruled out for the present study because
the XRD results have shown that there is not any change in
PEEK crystal structure.
From Fig. 8, it was observed that the Ton, Tc, and half
time of crystallization (t1/2) of PEEK was affected by the
presence of the n-Al2O3, which indicate that nucleation is
inhomogeneous. The addition of n-Al2O3 in PEEK shifts
the Tc towards higher temperature by 2–12 C depending
on the n-Al2O3 content in PEEK for a given cooling rate
in comparison to pure PEEK. This implies that the addition
of n-Al2O3 into PEEK enhanced the rate of PEEK crystallization. A similar enhancement of crystallization was
reported for AlN/PEEK [2], CaCO3/PP [13], SiO2/PP
[26,28,29], clay/PVDF [27], SiO2/PET [30,31], clay/PET
[32], and nanocomposites. However, our results are in contrast to the recent study of CaCO3/PEEK [5] and Al2O3/
PEEK [15] nanocomposites, where decrease in Tc was found
with the increase of fillers in PEEK matrix. This difference
may be attributed to the shape, size, loading, dispersion
level, adhesion, and surface morphology of the filler. Never-
1808
Table 4
The melting and crystallization data of n-Al2O3/PEEK nanocomposites
Sample
Tm (C)
Tc (C)
Ton (C)
DHc (J/g)a
vc
t1/2 (min)
DT (C)
NC-0
NC-1
NC-2
NC-5
NC-7
NC-10
NC-20
NC-30
334
335
336
335
338
340
337
336
270
268
276
272
275
273
279
283
284
277
288
285
285
283
289
290
31.83
30.18
27.04
29.19
30.35
29.56
28.73
32
24.46
23.22
20.8
22.45
23.35
22.74
22.10
24.62
2.8
2.0
2.4
2.6
2.0
2.0
2.0
1.4
64
67
60
63
63
64
56
53
a
Normalized heat of crystallization of PEEK constituent in nanocomposites.
theless the impurities present on the filler’s surface may also
affect the crystallization behavior of the polymer.
The half time (t1/2) of crystallization temperature of
PEEK nanocomposites was determined by using the equation [t1/2 = (Ton Tc)/rate of cooling]. Table 4 shows that
t1/2 value of nanocomposite decreases with the increase in
n-Al2O3 content in PEEK. The t1/2 for the pure PEEK is
2.8 min, which is decreased to about 1.4 min for the NC30 nanocomposites. The t1/2 for the nanocomposites varies
2.6–1.4 min depending on the nanofiller loading. The
decrease in t1/2 implies that the nucleation effect is
increased for PEEK with increase in n-Al2O3. For the same
rate of cooling, there is enough time for the molecular
chains of PEEK to pack into a closer arrangement.
Although the enthalpy of crystallization (DHc) for nanocomposites decreased slightly with the increase of n-Al2O3
as compared to the pure PEEK. Moreover, there is not
any trend in DHc with n-Al2O3 content. The supercooling
temperature (DT) of the nanocomposites decreases with
increasing n-Al2O3 in PEEK, indicating that the crystallization becomes easier in the nanocomposites due to the
nucleating effect of the n-Al2O3.
PEEK nanocomposites. The pure PEEK and PEEK constitute of nanocomposites crystallizes primarily in the form-I
[33] with orthorhombic crystal structure which shows diffraction peaks (2h) at about 18.7, 20.8, 22.9 and 28.9,
corresponding to diffraction planes of (1 1 0), (1 1 1),
(2 0 0), and (2 1 1). In the studied angular range for nanocomposites, there are only two weak diffraction peaks of
n-Al2O3 appearing at about 39.41, and 45.815, corresponding to Miller indices (2 2 2), and (4 0 0). Apparently,
apart from those of pure constitutes, no new diffracting
peaks were observed in the diffraction pattern of the nanocomposites. Moreover, all nanocomposite samples showed
the same XRD patterns with varying peak intensity in proportion of the constituent’s volume faction. The absence of
new diffraction peaks showed that the presence of n-Al2O3
did not change the crystal structure of PEEK. However, in
other polymer nanocomposite system a new diffracting
peak was observed which implies new morphology of the
polymer [27,34].
4.4. Crystal structure
Fig. 1a shows micrographs of pure PEEK powder at
2000· magnification. PEEK powders have irregular particles of rod like shape of length ranging from 10 to
50 lm. In order to determine the morphology of the nAl2O3 filler, it was dispersed in ethanol for 15 min under
ultra sonic bath and observed under the SEM. The fillers
are seen as agglomerates in Fig. 1b with sub-micron size
of primary particles, which are difficult to be resolved by
the SEM. This is due to the fact that n-Al2O3 particles have
a strong tendency to form agglomerate due to Wander
Wall’s forces between particle-particle. However the same
can be observed well separated in composites under SEM
due to interaction between n-Al2O3 and PEEK, which
results in well dispersion in PEEK matrix. Figs. 10a and
b show SEM micrographs for NC-1 and NC-10 after polishing. Due to the nano size, fillers are not distinctly visible.
In order to get distinct boundary between the n-Al2O3 filler
and the PEEK matrix, NC-1 and NC-10 nanocomposites
were etched in 2% w/v solution of potassium permanganate
in a mixture of 4 vol. of orthophosphoric acid and 1 vol. of
water. During etching amorphous PEEK or loosely
bounded PEEK surrounding the n-Al2O3 fillers were etched
Fig. 9 shows intensity versus angular position (2h) in the
range 10–50 of major crystallographic reflection for the
Fig. 9. X-ray diffraction pattern of the nanocomposites. For clarity, scans
of NC-1–NC-30 have been displaced upward.
4.5. Morphological examination
1809
Fig. 10. SEM micrographs of: (a) polished NC-1, magnification = 6 · 104; (b) polished NC-10, magnification = 6 · 104; (c) etched NC-1, magnification = 8 · 104; (d) etched NC-1, magnification = 1.6 · 105; (e) etched NC-10, magnification = 8 · 104; (f) etched NC-10, magnification = 1.6 · 105.
out, which results in appearance of n-Al2O3 fillers in PEEK
matrix. It could be seen from Figs. 10c–f that n-Al2O3 fillers were uniformly distributed throughout the PEEK
matrix. However, some n-Al2O3 agglomerates were also
seen in the PEEK matrix. Nevertheless, with increase of
n-Al2O3 content, the inter particle distance decreases which
results in formation of Al2O3 aggregates. As shown in Figs.
10c–f, SEM could not provide good contrast between nAl2O3 and PEEK matrix. For this reason, NC-1 and NC10 nanocomposites were also examined with TEM. Figs.
11a and b show TEM images of pure n-Al2O3 powder.
The n-Al2O3 particles are almost spherical in shape and
its size varies between 20 and 90 nm. Figs. 11c and d show
TEM images of NC-1 and NC-10 nanocomposites, respectively. The most of the n-Al2O3 particles remained individual in NC-1 nanocomposite. However, as the n-Al2O3
content increased to 10 wt% (NC-10) in PEEK, due to
the particle-particle interaction some aggregates of about
100 nm size was also observed with individual n-Al2O3 particles. This shows that shear forces applied during mechanical stirring were not capable of breaking and uniformly
distributing the n-Al2O3 in PEEK matrix.
1810
Fig. 11. TEM micrographs of: (a) n-Al2O3 powder as received, magnification = 6.6 · 104; (b) n-Al2O3 powder as received, magnification = 1.15 · 105; (c)
NC-1, magnification = 3.8 · 104; (d) NC-10, magnification = 3.8 · 104.
70
Experimental
ROM
Terner
Kerner
60
-6
The Tg of the PEEK determined by inflection in the
curve between dimension change and temperature was
found about 153 C. The average out-of-plane CTE
below Tg for the nanocomposites is shown in Fig. 12 as
a function of volume % of n-Al2O3 filler. The CTE of
the NC-0 was 58 · 106/C and decreased with increasing
n-Al2O3 filler in PEEK matrix. The CTE of the NC-30
(12 vol%) nanocomposite was about 23 · 106/C. The
reduction in CTE may be attributed to three reasons.
First, decrease in volume fraction of the PEEK in the
composite results in decreased free volume of PEEK,
hence reduced room for PEEK expansion. Second, well
dispersion of n-Al2O3 filler results in good interfacial area
between n-Al2O3 and PEEK. It is well known that in particulate polymer composites, particles are surrounded by
two regions; first by tightly bounded polymer or constrained polymer chain, and second, by loosely bounded
polymer chains or unconstrained polymer chain. As the
average inter-particle distance decreases with the incorporation of more filler particles, the loosely bound polymer
gradually gets transformed to the tightly bound polymer.
Hence, the volume fraction of loosely bound polymer
decreases [3,35]. Hence formation of increased con-
CTE (x 10 /˚C)
4.6. Coefficient of thermal expansion (CTE)
50
40
30
20
10
0
0
2
4
6
8
10
Volume % of n-Al 2O3 in PEEK
12
14
Fig. 12. Theoretical and experimental CTE of the nanocomposites as a
function of the n-Al2O3 content.
1811
strained PEEK suppresses the thermal expansion of the
nanocomposites. Third, probably due to the much lower
intrinsic CTE of Al2O3 (6.6 · 106/C) as compared to
pure PEEK (58 · 106/C).
Various models such as rule of mixture (ROM), Turner,
and Kerner’s models have been discussed in literature for
obtaining the CTE of composites. The simplest model for
CTE of the composite material is the ROM, which serves
as the first order approximation to the overall calculation
of the CTE of the composite. This can be expressed as
ac ¼ am ð1 V f Þ þ af V f
ð5Þ
where ac, am, and af represent the CTE of the composite,
matrix, and filler, respectively. As shown in Fig. 12, the
ROM overestimates the CTE for the composites as compared to the experimental CTE. This is due to the fact that
it does not take into account the mechanical constraint created on the matrix due to fillers. However, Turner model
takes into account the mechanical stress on adjacent phases
in the composites [36–38]. This can be expressed as
ac ¼ ðam V m Y m þ af V f Y f Þ=ðV m Y m þ V f Y f Þ
ð6Þ
where Ym and Yf are the bulk modulus of the matrix and
filler phase, respectively. This model predicts CTE lesser
than the values obtained from the ROM and the experimental CTE. Turner model estimates the CTE of composite based on the bulk modulus of filler and matrix, but the
bulk modulus of the Al2O3 filler (247 GPa) is much larger
(i.e. two order of magnitude higher) than that of the PEEK
matrix (3.6 GPa). Thus CTE of the composites is closer to
the CTE of the filler.
Kerner has developed the following expression for the
CTE of composite consisting approximately spherical particles dispersed in matrix and wetted by a uniform layer of
matrix. Composites were assumed to be macroscopically
isotropic and homogeneous [38].
ac ¼ am V m þ af V f ðam ac ÞV m V f K
ð7Þ
where
K ¼ ½ð1=Y
40 nm will have one million times number of nano particles
than that of conventionally used filler of 4 lm [40]. Hence,
nanofillers are more effective in reducing the CTE of the
matrix.
Presently commercial glass/epoxy (FR-4) composite is
used as packaging substrates in electronics, which have
much higher out-of-plane CTE (>60 · 106/C) than that
of copper (18 · 106/C) [41]. The CTE of the copper must
match with that of the packaging substrate materials to
avoid the thermal fatigue failure. In the present study,
the decrease in CTE with increase of n-Al2O3 indicates better dimensional stability of the composites as compared to
pure PEEK, making them potential candidate for electronic packaging materials. The novelty of the present
work is that the desired CTE of the nanocomposites was
achieved at much lower filler loading (12 vol%) as compared to the reported filler loading [7,11]. For example
CTE of the epoxy is reduced from 88 · 106/C to
40 · 106/C at 50 vol% Al2O3 (12–15 lm) loading [7]
and of polyvinylidene fluoride (PVDF) is reduced from
137 · 106/C to 60 · 106/C at 50 vol% AlN (1.5 lm)
loading [42].
5. Conclusions
The PEEK matrix nanocomposites reinforced with nAl2O3 filler showed improvement in thermal stability and
CTE. The thermal stability of nanocomposites is improved
in both, i.e. air and nitrogen atmosphere which may be
attributed to the interaction between the PEEK matrix
and the n-Al2O3 fillers. The peak crystallization temperature and melting temperature of the nanocomposites were
increased significantly. The CTE of NC-30 nanocomposite
was decreased to 40% of the pure PEEK, which is very
close to the CTE of copper. Hence, these nanocomposites
may be the suitable futuristic electronic packaging substrates in electronic/microelectronic applications.
1
m
1=Y f ÞðV f =Y m þ V m =Y f þ 3=4:Gm Þ where Gm is the shear modulus of the matrix, Ym, and Yf
are the bulk modulus of the matrix and filler, respectively.
The interaction term K is a measure of the thermal stress
occurring in the composite systems during temperature
changes. This equation differs from the ROM by the last
term because the fillers constraint the matrix. Hence, this
equation predicts CTE lesser than the values predicted by
ROM, but predicts higher than the experimental CTE. This
discrepancy may be accounted for the large surface area to
volume ratio of n-Al2O3 filler, which constraint more
PEEK matrix fraction, as compared to conventionally used
micron size fillers as models are developed for latter. The
CTE of the PEEK composite reinforced with 12 vol% micro-Al2O3 (size: 8 lm) was decreased to 37 · 106/C [39],
which is much higher than that of PEEK reinforced with
12 vol% nano-Al2O3. For example, for a constant filler
loading in a same volume of composite, nanofiller of size
Acknowledgements
We thank Dr. P.D. Trivedi, Polymer Division, Gharda
Chemicals, India for providing PEEK powder for this research work. We also thank Dr. S.L. Kamath, IIT Bombay
for performing DSC analysis and Mrs. Anuya Nisal, NCL
Pune for making ultra-thin composite section. SAIF, IIT
Bombay is acknowledged for the TEM analysis. We are
grateful to Dr. T. L. Prakash, Executive Director of CMET for his interest in this work.
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